Polymer light-emitting diodes (PLEDs) which employ semiconducting polymers as emitting layers have been demonstrated. A wide range of colors of emission can be achieved by varying the materials present in the emitting layers. Blends of emitting polymers alone and together with organometallic emitters can be used to achieve additional color shades of emitted light including white light.
LEDs that emit white light are of interest and potential importance for use as back lights in active matrix displays (with color filters) and because they can be used for solid state lighting [A. J. Heeger, Solid State Commu., 1998. 107,673 & Rev. Modem Phys., 2001,73, 681; B. W. D'Andrade, S. R. Forrest, Adv. Mater., 2004, 16, 1585; R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. J. Ögdlund, and. W. R. Salaneck, Nature, 1999, 397, 121]. In such applications, the fabrication of large-area devices and the use of low-cost manufacturing technology will be the major issues. The fabrication of PLEDs by processing the active materials from solution (e.g. by use of ink-jet printing or other printing technologies) promises to be less expensive than that of OLEDs (organic light-emitting diodes based on small molecules) where deposition of the active layers requires the use of vacuum technology [B. W. D'Andrade, S. R. Forrest, Adv. Mater., 2004, 16, 1585] Several approaches have been used to generate white light and light of other colors from OLEDs and PLEDs [J. Kido, H, Shionoya, K, Nagai, Appl. Phys. Lett., 1995, 67, 2281-2283; C. Zhang, A. J. Heeger, J. Appl. Phys., 1998, 84, 1579; Z. Shen, P. E. Burrows, V. Bulvić, S. R. Forrest, M. E. Thompson, Science, 1997, 276, 2009; Y. Hamada, T. Sano, H. Fujii, Y. Nishio, Jpn. J. Appl. Phys., 1996, 35, L1339; Y. Z. Wang, R. G. Sun, F. Meghdadi, G. Leising, A. J. Epstein, Appl. Phys. Lett., 1999, 74, 3613; M. Strukelj, R. H. Jordan, A. Dodabalapur, A.; J. Am. Chem. Soc., 1996, 118, 1213; B. W. D'Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater., 2004, 16, 624]. In the approaches in the articles listed above, the efficiency was modest and the lifetime was limited by that of the blue emitters [J. Kido, H, Shionoya, K, Nagai, Appl. Phys. Lett., 1995, 67, 2281; Y. Hamada, T. Sano, H. Fujii, Y. Nishio, Jpn. J. Appl. Phys., 1996, 35, L1339; Y. Z. Wang, R. G. Sun, F. Meghdadi, G. Leising, A. J. Epstein, Appl. Phys. Lett., 1999, 74, 3613; M. Strukelj, R. H. Jordan, A. Dodabalapur, A.; J. Am. Chem. Soc., 1996, 118, 1213; U. Scherf, E. J. W. List, Adv. Mater. 2002, 14, 477; S. Setayesh, D. Marsitzky, K. Müllen, Macromolecules, 2000, 33, 2016; X. Gong, P. Iyer, D. Moses, G. C. Bazan, A. J. Heeger, Adv. Func. Mater., 2003, 13, 325].
PLEDs fabricated with semiconducting polymers doped with organometallic emitters offer the additional promise of “plastic” electronics. Representative examples of such devices are described in U.S. application Ser. No. 10/680,084 filed Oct. 3, 2003. The emissive layers of PLEDs can be fabricated by casting polymers and blends from solution, thereby enabling relatively simple and low cost manufacturing processes [G. D. Müller, A. Falcou, N. Reckefuss, M. Roijhn, V. Wiederhimggg, P. Rudati, H. Frohne, O. Nuyken, H. Becker, K. Meerholz, Nature, 2003, 421, 829].
The fabrication techniques most favored for producing multilayer PLEDs include the use of sputtering and various vapor deposition methods to lay down inorganic layers such as high work function metal-metal oxide contacts (electrodes) and protective metallic overlayers. Solvent deposition methods such as spin-casting or printing successive layers from solution can be used to lay down organic polymer emissive layers as well as other layers in the devices. When multiple organic layers are present there can be problems with successive layers interacting. The solvent of a later layer can dissolve or disfigure (etch) a prior layer. It is often desirable to have each layer be smooth and coherent, thus this interaction can be destructive.
Light may be characterized by three quantities: the CIE (Commission Internationale de l'Eclairage) coordinates, the color temperature (CT) and the color rendering index (CRI). “Pure” white light has CIE coordinates of (0.333, 0.333), and is obtained by balancing the emission of the colors employed. For illumination applications, the CT needs to be equivalent to that of a blackbody source between 3000°K. and 7500°K. Average daylight has CT=6500°K., while a fluorescent lamp (warm white) has CT=3000°K. [R. W. G. Hunt, Measuring Color, 2nd Ed. Ellis Horwood, 1991]. The CRI is a numerical measure of how “true” colors look when viewed with the light source. CRI values range from 0 to 100, with 100 representing true color reproduction. Fluorescent lamps have CRI ratings between 60 and 99. Though a CIU value of at least 70 may be acceptable for certain applications, a preferred white light source will have a CRI of about 80 or higher. The demonstration of PLEDs which emit illumination quality white light with high brightness, high efficiency, suitable CT, high CRI and stable CIE coordinates is of importance to the future of solid state lighting.